1. Field of the Invention
The present invention relates to a hydraulic control system controlling hydraulic pressure that is supplied to hydraulic frictional engagement elements provided in an automatic transmission.
2. Discription of the Prior Art
An automatic transmission for a vehicle supplies hydraulic pressure to hydraulic frictional engagement elements such as clutches and brakes, and automatically performs change of gear ratio (gear speed) between an input shaft and an output shaft according to driving states of the vehicle by engaging desired rotating elements such as rotating drums and gears, by engaging or the rotating elements and fixed elements such as cases by means of these hydraulic frictional engagement elements.
In this automatic transmission, if increase of transmission torque of the hydraulic frictional engagement elements, that is, so-called engagement elements that change from released state to engaged state through upshift is improper, an uncomfortable shock is generated during the shifting.
For example, if increase of engaging capacity of the engagement elements during upshift is too rapid, torque at the time of start of an inertia phase after a torque phase is terminated rises rapidly as shown by an output shaft torque waveform of a broken line F in FIG. 9, and an uncomfortable shift shock is generated.
Moreover, if increase of the engaging capacity of the engagement elements during upshift is too gradual, both the torque phase and inertia phase are prolonged as shown by an output shaft torque waveform of a broken line G in FIG. 9, and an extraordinary long time is required till the upshift is finished. This causes not only an uncomfortable body feeling but also increase of heating amount of the hydraulic frictional engagement elements during the shifting, whereby durability is deteriorated.
An example of devices preventing such shift shock and slow feeling is described in Japanese Laid-Open Patent Publication No. 62-41460. This device makes change characteristic of hydraulic pressure that is supplied to the engagement elements proper and performs hydraulic control so that an output shaft torque reaches an adequate value for the inertia phase without thrust-up at the time of start of the inertia phase after the torque phase as shown by a solid line E in FIG. 9.
Moreover, if the increase of the engaging capacity of the engagement elements during the torque phase is further hastened, and the increase of the engaging capacity of the engagement elements at the time of start of the inertia phase is appropriately set, a decrease time (hereinafter referred to as a pull time) of torque of the torque phase is shortened.
If this pull time is made shorter than a predetermined time, torque reduction due to the torque phase cannot be completely transmitted to a drive system provided downstream from the automatic transmission in relation to frequency response of the drive system.
That is, if torque phase time is shortened as shown by an output shaft torque waveform of a solid line E in FIG. 10, shock generated in acceleration of the vehicle at the time of torque phase takes a form as annealed in response to change of the output shaft torque, like acceleration of the vehicle as shown by a thick line H in FIG. 10, and a pull-in shock due to the torque phase can be reduced.
Thus, by controlling timing of release or engagement of the engagement elements so that time of the torque phase is shortened while shock of the inertia phase of the output shaft torque as shown by the solid line E in FIG. 9 is maintained as it is, a comfortable shift feeling including a good accelerating feeling and reduced shift shock can be obtained.
However, in the conventional device, even if timing of engagement or release of the brakes and clutches is controlled so that the output shaft torque waveform as shown by the solid line E in FIG. 10 is established in order to obtain a comfortable shift feeling, characteristics of hydraulic pressure, μ characteristics of friction material used for the engagement elements, dispersion of engine torque and the like complicates the stable realization of the comfortable shift feeling.
For example, if the characteristics of hydraulic pressure is low, μ is low, or the engine torque is large, that is, dispersion occurs in the direction where transmission torque of the engagement elements is small compared to the input torque, the torque phase does not readily proceed, and therefore the pull time of the torque phase cannot be shortened.
Moreover, if the characteristics of hydraulic pressure is high, μ is high, or the engine torque is small, that is, dispersion occurs in the direction where the transmission torque of the engagement elements is large compared to the input torque, the inertia phase proceeds at an engagement capacity wherein termination of the torque phase is supposed. Since it is aimed to raise the capacity at a dash till the termination of the torque phase, if the inertia phase starts at such a rising gradient, a great shock occurs at the time of start of the inertia phase as shown by a broken line F in FIG. 9.
If rise of hydraulic pressure during the torque phase is not made extraordinarily rapid, the above effect (pull shock of the torque phase can be reduced) cannot be obtained. Actually, unless a command value of hydraulic pressure that is commanded to a hydraulic control system is set to a value by which hydraulic pressure is rapidly raised after a piston stroke of the engagement elements is terminated, and hydraulic pressure is quickly raised to a maximum, the above-described shock cannot be realized.
Considering responsibility and stability until transition to the torque phase after a piston stroke time and a piston stroke are terminated, it is necessary to set a hydraulic pressure command value approximately to a hydraulic pressure by which the inertia phase is started while the engagement elements are stroked by piston, and to raise the hydraulic pressure stepwise from immediately after termination of the piston stroke.
Therefore, it is necessary to appropriately command a so-called piston stroke control command pressure that is a hydraulic pressure value fully progressing the piston stroke to a hydraulic control unit so as to meet the above purpose. However, when dispersion factor (friction coefficient of friction material of the engagement elements or hydraulic pressure) that has influence on the transmission torque of the engagement elements disperses in a wide range, or when dispersion factor (engine torque) that has influence on the input torque disperses in a narrow range, since hydraulic pressure rises stepwise, compared to the case when hydraulic pressure rises gently, a rapid thrust-up shock that is caused because the output shaft torque rises more rapidly than aimed at the time of start of the inertia phase is remarkably apt to occur. For this reason, it is extremely difficult to set the piston stroke control command pressure that fully exerts effect that restrains generation of such a thrust-up shock and simultaneously shortens torque phase time.